In the course of increasingly stringent environmental guidelines, the sensory system in the region of combustion-engine exhaust gases is becoming increasingly important. In this context, particularly gas sensors based on solid electrolyte are used, which identify the gaseous components to be detected in the exhaust gas in a highly selective manner. In combustion engines operated with a hyperstoichiometric amount of air, the formation of nitrogen oxides is increased. In order to be able to remove them from the exhaust gas, emission-control systems based on the addition of reductive substances to the exhaust system are important. For example, ammonia or ammonia-releasing substances such as urea are used for this purpose. Since besides the emission of nitrogen oxides, the release of ammonia into the environment should also be prevented, the demand for gas sensors, which are suitable for determining both ammonia and reducible gases such as nitrogen oxides, is increasing.
German patent document no. 199 30 636 discusses and refers to a solid-electrolyte-based gas sensor, which is used to detect nitrogen oxides. The measuring principle of the sensor is based on the fact that, inside the gas sensor, excess oxygen is removed without affecting the nitrogen-oxide concentration, a constant, low oxygen concentration being set at a first pump electrode. Then, first of all, the concentration of remaining oxygen and, secondly, the sum of the concentrations of remaining oxygen and nitrogen oxides are concurrently determined in an amperometric manner. The nitrogen-oxide concentration is simple to calculate by forming the difference of the two signals. However, this sensor is limited to determining the concentration of oxygen and nitrogen oxides.